Powder coatings for architectural aluminium

Uncover the secrets of powder coatings for architectural aluminium. Learn about the composition and characteristics of powder paints, including binders and pigments.

Coating Powder Composition

Coating powder consist of four types of components [2]:

• binding polymeric materials and their hardeners
• pigments
• fillers
• supplements.

The chemical composition of these components of the powder paint and the ratio of their number determine the characteristics of the finished powder coating.

Binder material consists of one or more organic resins, as well as hardener. Together they determine the main characteristics of polymer powder paint (reactivity, ability to form a coating, mechanical properties and so on).

The type of paint is determined by the main type of binder organic resin. Currently, the following types of organic resin binders are used [1, 2]:

• epoxy
• polyurethane
• acrylic
• hybrid (for example, epoxy-polyester)
• polyester
• fluoropolymer.

In addition to these organic resins and their hardeners, powder paints also contain:

• Pigments. May be organic, inorganic or metallic. They affect the color and appearance of the coating surface., as well as the durability of the coating.
• Fillers. Improve rheological or chemical properties of the polymer coating.
• Supplements. Small amounts are added to the polymer powder material to improve some characteristics of the polymer coatings., for example, gloss.

Types of Powder Coatings

Appointment of various types of powder paints [1]:

• Epoxy powder paints are designed for products., which require high chemical and corrosion resistance, as well as attractive appearance, subject to minor environmental influences.
• Polyurethane powder paints exhibit properties, which are similar to the properties of epoxy paints, but have higher resistance to climatic conditions.
• Acrylic powder paints are used mainly in the form of transparent coatings..
• Hybrid powder coatings are a compromise between epoxy and polyester coatings. They are produced in various ratios of these types of polymers, depending on the special requirements for finished coatings..
• Polyester Powder Coatings Specially Designed for Outdoor Use, including as architectural powder coatings on buildings and structures of general purpose.
• Fluoropolymer powder coatings provide the highest level of resistance to climatic and other environmental influences, which are currently available. Fluoropolymer powder coatings are used at prestigious and monumental architectural sites., which require long-term preservation of high decorative quality.

Powder coating of aluminium

Vertical powder coating

European Standard EN 12206-1

This document specifies requirements and corresponding methods of test relating to the organic coating of aluminium and aluminium alloy extrusions, sheet and preformed sections for archtectural purposes, using coating powders.

Industry Standards

Currently, several international industry standards are operating in the world, which regulate the use of powder coatings for architectural aluminum.  Currently, the most famous and widely used standards for architectural powder coatings are:

• GSB AL 631
• Qualicoat Specifications
• AAMA 2603, AAMA 2604 AAMA 2605
• HG/T 3793.

Figure 1 shows the countries of origin of each of these standards and a list of countries and regions, where these standards are mainly applied.

  

Figure 1 – Basic international standards
for architectural powder coatings:
country of origin, countries and regions of application [3]

Durability: gloss and color

Residual gloss

The figure 2 summarized requirements, which are specified in these standards for full-scale testing of coatings in southern Florida. For each durability grade (Florida 1, 3, 5 or 10) the minimum permissible gloss retention (residual gloss) is set as a percentage. Moreover, Qualicoat sets minimum gloss levels for multiple test points during durability tests. for instance, for Class 3, required, to:

• after 1 years in Florida, the residual gloss was more 90 %;
• after 4 years – more 70 %;
• after 7 years – more 55 % и
• after 10 years – more 50 %.

Figure 2 – Classification of powder coatings
by the duration of field tests in Florida:
minimum gloss requirements
according to various standards for architectural powder coatings [3]

Unlike GSB standards, Qualicoat and AAMA Chinese standard HG / T 3793-2005 Doesn’t Set Florida Test Requirements, a takes into account only the results of accelerated durability tests using xenon light (Figure 3).

Figure 3 – Artificial accelerated powder coating tests
according to various standards for architectural powder coatings [3]

In the drawing table 3 shows the requirements of standards for artificial accelerated testing for durability and the correlation of the requirements of these artificial tests with field tests in the conditions of southern Florida. It is seen, that different standards apply very different methods to assess the durability of coatings. Some of them require shorter test times., giving obvious benefits to test coatings, but it reduces the degree of their correlation with field tests [3]:

• GSB standard establishes tests using fluorescent ultraviolet radiation, which are the fastest and cheapest.
• Qualicoat и HG/T 3793-2005 apply tests with irradiation of samples with xenon light. It is believed, that the xenon test more accurately simulates the full spectrum of natural sunlight, which gives a good ratio between artificial and field tests for durability.
• AAMA does not establish any artificial accelerated durability tests, and relies only on field tests in Florida.

Color change

All four standards set the maximum color deviation limit after durability tests., artificial or natural (drawing 4). GSB and Qualicoat set the maximum color deviation for each of the RAL colors., whereas AAMA and HG / T 3793-2005 set the total tolerance for all colors.

Figure 4 – Maximum allowable color change
according to various standards for architectural powder coatings [3]

Powder Coating Durability

Typically, powder coatings are classified into:

• coatings for use inside buildings;
• coatings for outdoor use, but no decorative requirements;
• coatings for outdoor use on products, details and designs, which are architectural components.

obviously, that coatings for external use should show much higher UV resistance, than coating brands for internal use, which are exposed to minor UV radiation, for example, through the windows.

Brand coatings for external use are damaged mainly by strong ultraviolet radiation, often in combination with moisture. Moisture is especially critical in the presence of hydroxyl radicals., which are formed under the influence of ultraviolet radiation. These radicals react chemically with organic coating polymers., which leads to its degradation [1].

Not all RAL colors

There are certain restrictions on the available RAL colors for “super durable” and “hyper durable” powder coatings. So, of 190 RAL colors, with which Qualicoat works, 12 colors cannot meet the durability requirements for the Class 2, a 67 colors – for Class 3 [2].

The specifications of powder coating on architectural aluminium

The durability of a powder coating does not only depend on the category of powder paint. Therefore, in the contract for the supply of aluminum architectural products and structures, especially for large, dear, prestigious or monumental architectural objects, usually include, including, the following provisions, requirements and conditions [4-6]:

• Durability category of powder coating: for example:
  – Class 1, 1.5, 2 or 3 по Qualicoat
  – Standard, Master or Premium by GBS.
• Type of chemical surface preparation: “yellow” chromation, “Green” chromation, chromate-free preparation, preparation by anodizing).
• Building type: non-residential, residential, low-rise, multi-storey, office, prestigious, monumental.
• Geographic location of the facility under construction.
• Climatic zone of the object (continental, seaside, urban, rural)
• Color according to the approved catalog, for example, RAL.
• Gloss category.
• Requirements for color deviation and gloss within a coated lot or between lots.
• Recommendations for repairing damage to the coating, that may arise during construction.

Sources:

1. The Architect’s Guide to Powder Coating – www.ifscoatings.com
2. Qualicoat Specifications
3. Material Testing Product and Technology News – Volume 42 – Issue 92 – 2012
4. Guide to correct specifcation of powder coated aluminium – Qualicoat Guidance Note – No.2 – July 2019
5. How to Specify / Ben Parsley – Qualicoat Presentation Oct 2017
6. How to get a Dulux Powder Coat Warranty on your project / architectureanddesign.com.au